A torque transducer having a rotating shaft with first and second substantially rigid torque input members connected by a torsionally compliant coupling that enables angular deflection of the first torque input member relative to the second torque input member as a function of the magnitude of the torque in the shaft. first and second grating elements are attached to or integral with first and second torque input members, respectively. The grating elements have surfaces with alternating regions of high and low reflectivity connected by a torsionally compliant coupling. The surfaces are illuminated by a source of electromagnetic radiation (emr), which generates patterns on one or more arrays of detectors sensitive to the emr. The disposition of the patterns is a function of torque applied to the shaft, and the output of the one or more arrays can be processed to produce a measure of the torque applied to the shaft.
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1. A torque transducer comprising a rotating shaft at least partially surrounded by a fixed housing, an axis of rotation of the shaft fixed with respect to the housing, the shaft comprising first and second torque input members, which are connected by a torsionally compliant coupling, the coupling enabling angular deflection of the first torque input member relative to the second torque input member as a function of a magnitude of the torque in the shaft, a first grating element attached to or integral with the first torque input member and a second grating element attached to or integral with the second torque input member, the first grating element comprising a first surface and the second grating element comprising a second surface, the transducer also comprising at least one electromagnetic radiation (emr) source and at least one array of emr sensitive detectors, wherein said source irradiates one or both of the surfaces and said array receives incident emr reflected from one or both of the surfaces, the at least one source irradiating each surface and the at least one array receiving incident emr reflected from this surface are all positioned on the same side of this surface and fixed with respect to the housing, both surfaces comprise alternating regions of high and low reflectivity, a pattern produced by incident emr of the at least one array at any instant of time resulting from the alternating regions of low and high reflectivity on the one or both surfaces providing reflected emr to this array, regardless of the angular position of the shaft and irrespective of the relative angular deflection of the first and second torque input members, the output from the at least one array, resulting from the pattern on the at least one array at said any instant of time, is processed by a processor to derive the relative angular deflection of the first and second torque input members, and hence provide a measure of the magnitude of the torque in the shaft.
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This invention relates to torque transducers for measuring the magnitude of torque in shafts, in particular rotating shafts such as found in electric power steering systems in vehicle applications.
Electric power steering systems conventionally incorporate an input shaft element, connected via an intermediate shaft and Hookes joint arrangement to the steering wheel. The input shaft therefore needs to rotate through an angle typically one to two revolutions either side of the on-centre steering position. The input shaft is at least partially surrounded by the fixed housing of the steering gear. It is a requirement of the electric power steering servo system to accurately measure the continuously varying torque in this rotating shaft. Conventionally torque applied to the shaft causes it to is angularly deflect, such deflection causing one part of the shaft to angularly displace with respect to another part, and this displacement is sensed to provide a measurement of this torque.
The sensing means needs to allow for rotation of the shaft within the housing, usually employing non-contact or mechanical signal transmission means. Non-contact means include optical aperture based devices and magnetic devices such as magnetostrictive or variable reluctance couplings. Mechanical means include slidably connected potentiometers and other indicating devices.
To improve the accuracy of such sensing means a torsionally compliant coupling in the form of a torsion bar is used to connect the two input members at either end of the shaft When torque is applied between the two input members the torsion bar deflects causing an increased angular displacement, which allows the use of less sensitive, or less accurate sensing means.
The torsion bar may be in the form of a separate element as in the case of a conventional rotary hydraulic power steering valve. Alternatively, in the case of some proposed electric power steering systems, the torsion bar may in fact be integral with the shaft member and be a relatively torsionally compliant (ie. less torsionally stiff) portion of the shaft member which couples substantially rigid torque input members at each end of the shaft member. The shaft member in these latter systems can be readily machined as a single steel component, and the only requirement is that the angular deflection of the relatively torsionally compliant coupling portion, connecting the two substantially rigid torque input member portions, has sufficiently low torsional stiffness that the sensing system is able to accurately measure its angular deflection.
Generally, the use of a torsion bar requires the use of a failsafe mechanism, being a torque limiting device to prevent failure of the torsion bar when unavoidable torque overload conditions occur.
Such torque limiting devices are well known in the art of vehicle steering, and will therefore not be described in this specification.
The prior art, which is most closely related to that of the present invention, is described in U.S. Pat. No 5,369,583 and International Patent Application PCT/GB95/02017 which show sensors employing optical disc apertures for measuring torque.
The essence of the present invention resides in the provision of grating elements comprising surfaces composed of alternating regions of high and low reflectivity connected by a torsionally compliant coupling. These surfaces are illuminated by a source of electromagnetic radiation (EMR), typically UV, visible or IR light, which generates patterns on one or more arrays of detectors sensitive to the EMR. Arrays include CCD devices, VLSI vision chips, one and 2 dimensional photodetector arrays and lateral effect photodiodes (commonly referred to as PSD's or position sensitive devices). The disposition of the patterns is a function of torque applied to the shaft, and the output of the one or more arrays can be processed to produce a measure of the torque applied to the shaft. It is distinguished from other reflective torque transducers by use of an reflective imaging approach which does not rely on Moire fringes, speckle patterns or other diffraction gratings. As it uses photo detector arrays, EMR reflected from the gratings provide an instantaneous image which allows a much faster and more complete means of interpreting the information than is possible with individual photo-detectors. In the latter case it is necessary to count successive changes of EMR intensity incident on the photo-detector, which is slower and more prone to error.
Another reflective torque transducer that uses arrays is described in U.S. Pat. No. 5,490,430. This relies on a change in diffraction angle of two or more diffraction gratings that are torsionally strained by the application of torque. This device is prone to error due to misalignment and bending load and requires a collimated and monochromatic source of EMR. The regions of high and low reflectivity can be arranged axially or radially about the axis of rotation of the shaft, and are of such a nature that allows a continuous output of the arrays at any instant in time regardless of the angular position of the shaft, as the limited array dimensions may not allow the complete circumference or radial face to be viewed by the arrays. The advantages of such a construction over that disclosed in U.S. Pat. No. 5,369,583 and International Application Number PCT/GB95/02017 may arise as one or more of the following:
Firstly, the use of reflective grating elements allows simpler and more compact construction by the use of a cylindrical grating element arrangement, which is not readily achievable using disc apertures as shown in the prior art without requiring a significantly increased diameter. It also allows the EMR source(s) and array(s) to be packaged in the same assembly with further savings in space and cost. Secondly, it allows for easy assembly and disassembly of the transducer, as the grating elements can be removed from one end of the transducer in an axial direction without disturbing the EMR source(s) or array(s).
Thirdly, another advantage with the use of reflective grating elements is that the EMR is reflected from the surface, and is not affected by edge scattering as is the case with apertures with a non-zero thickness. Such scattering limits the maximum resolution of the device. Fourthly, the use of reflective grating elements allows the use of well known and accurate photographic or metallising techniques, for example metal on glass. The use of these techniques with apertures may result in loss of resolution or other problems from internal reflection, diffraction or degradation over time as the EMR has to travel through the glass between the metallised regions.
Finally, the use of reflective grating elements allow the use of intermeshed castellations which can provide a lost motion connection limiting the maximum angular deflection of the torsion bar, thereby eliminating the need for a separate torque limiting device and reducing the cost and complexity of the transducer.
The present invention consists in a torque transducer comprising a rotating shaft at least partially surrounded by a fixed housing, the axis of rotation of the shaft fixed with respect to the housing, the shaft comprising first and second substantially rigid torque input members which are connected by a torsionally compliant coupling, the coupling thereby enabling angular deflection of the first torque input member relative to the second torque input member as a function of the magnitude of the torque in the shaft, a first grating element attached to or integral with the first torque input member and a second grating element attached to or integral with the second torque input member, the first grating element comprising a first surface and the second grating element comprising a second surface, the transducer also comprising one or more electromagnetic radiation (EMR) sources and one or more arrays of EMR sensitive detectors, characterised in that each source irradiates one or both of the surfaces and each array receives incident EMR reflected from one or both of the surfaces, the one or more sources irradiating each surface and the one or more arrays receiving incident EMR reflected from this surface are all positioned in the same side of this surface and fixed with respect to the housing, both surfaces comprise alternating regions of high and low reflectivity, a pattern produced by incident EMR on each of the one or more arrays at any instant of time resulting from the alternating regions of low and high reflectivity on the one or both surfaces providing reflected EMR to this array regardless of the angular position of the shaft and irrespective of the relative angular deflection of the first and second torque input members, the output from the one or more arrays, resulting from the pattern or patterns on the one or more arrays at said any instant of time, is processed by a processor to derive the relative angular deflection of the first and second torque input members, and hence provide a measure of the magnitude of the torque in the shaft.
In some embodiments of the present invention a first array receives incident EMR reflected from a first surface and results in a first pattern, and a second array receives incident EMR reflected from a second surface and results in a second pattern. It is preferred that the processor receives inputs from the first and second arrays, and the processor comprises software or hardware electronic means to determine the relative displacement of the first and second patterns.
In other embodiments of the present invention the first and second surfaces are either mutually adjacent or contiguous, a single array receives incident EMR reflected from both first and second surfaces and results in a single pattern, the pattern comprises a is first subpattern produced by the incident EMR reflected from the first surface and a second subpattern produced by the incident EMR reflected from the second surface. It is preferred that the processor receives inputs from the single array, and the processor comprises software or hardware electronic means to determine the relative displacement of the first and second subpatterns. It is preferred that the single pattern is an interdigital pattern comprising the first subpattern interposed between the second subpattern.
It is preferred that at least one of first or second surfaces is substantially cylindrical with a central axis collinear with the axis of rotation of the shaft, and the array, which receives incident EMR reflected from the at least one surface, is positioned radially inside or outside the surface. It is preferred that the at least one substantially cylindrical surface is discontinuous due the respective grating element comprising radially protruding castellations around its periphery, the castellations are substantially axially aligned, the regions of high reflectivity correspond to the areas of maximum radius of the castellations with respect to the central axis of the cylindrical surface, and the regions of low reflectivity are angularly aligned with the discontinuous gap areas or lesser radius areas between the castellations. Also it is preferred that the grating element is manufactured from metal or plastic material and the areas of maximum radius are smoothly machined, moulded or sintered, or surface treated with paint or material deposition to impart high reflectivity, and the discontinuous gap areas or lesser radius areas are machined, moulded or sintered, or surface treated with paint or material deposition to impart low reflectivity.
Alternatively, in certain applications, it may be preferred that the at least one substantially cylindrical surface is substantially continuous due to the respective grating element comprising a substantially smooth cylinder, the inside or outside surface of the cylinder comprising the alternating regions of high and low reflectivity, and the regions are substantially axially aligned. Preferably the regions of high reflectivity are. metallised, shiny or light coloured and the regions of low reflectivity are substantially transparent, roughened or dark coloured.
Alternatively, in certain applications, it may be preferred that the at least one of first or second surfaces is substantially radially disposed with respect to the axis of rotation of the shaft, and the array, which receives incident EMR reflected from the at least one surface, is positioned axially on one side of the surface. Preferably, the at least one substantially radially disposed surface is discontinuous due to the respective grating element comprising axially protruding castellations around its periphery, the castellations are substantially radially disposed, the regions of high reflectivity correspond to the areas of maximum axial protrusion of the castellations, and the regions of low reflectivity are angularly aligned with the discontinuous gap areas or less axially protruding areas between the castellations. Also it is preferred that the grating element is manufactured from metal or plastic material, the areas of maximum axial protrusion are smoothly machined, moulded or sintered, or surface treated with paint or material deposition to impart high reflectivity, and the discontinuous gap areas or less axially protruding areas are machined, moulded or sintered, or surface treated with paint or material deposition to impart low reflectivity.
Alternatively, in certain applications, it may be preferred that the at least one substantially radially disposed surface is substantially continuous due to the respective grating element comprising a substantially smooth disc or planar ring, one side of the disc or planar ring comprising the alternating regions of high and low reflectivity, the regions are substantially radially disposed, the regions of high reflectivity are metallised, shiny or light coloured, and the regions of low reflectivity are substantially transparent, roughened or dark coloured.
Preferably the array comprises a one dimensional or a two dimensional array, a CCD, a VLSI vision chip or a lateral effect photodiode.
Preferably the pattern or patters is also processed by a processor to derive angular velocity and/or the relative angular position of at least one of the torque input members Preferably surface of at least one grating element includes areas or additional regions of high or low reflectivity whose resulting pattern is also processed to derive absolute angular position of the torque input member to which the at least one grating element is attached to or integral with.
Preferably the alternating regions of high and low reflectivity on the surface of the at least one grating element are arranged in the form of a succession of individual binary bar codes arranged such that the individual bar codes do not overlap. Alternatively the alternating regions of high and low reflectivity on the surface of the at least one grating element are arranged in the form of a succession of individual bar codes arranged such that the individual bar codes overlap. The resulting pattern on the respective array is processed to derive the absolute angular position of the torque input member to which the at least one grating element is attached to or integral with. It is preferred that a succession of binary bar codes are employed on both grating elements and the difference in the absolute angular position of the first and second torque input members is used to provide a measure of the magnitude of the torque in the shaft.
Preferably the first and second grating elements are adjacent and comprise radially extending intermeshing castellations, clearance being provided between the castellations and thereby providing a rotational lost motion connection between the first and second torque input members and hence limiting the maximum angular deflection of the torsionally compliant coupling.
The present invention will now be described by way of example with reference to the accompanying drawings, in which:
Failsafe mechanism 15, shown in cross section in
For all six embodiments described above the pattern migrates across the limited width one dimensional or two dimensional array(s) as the shaft rotates, quite independent of shaft torque. Again, using techniques well known in the discipline of pattern recognition, the rate of pattern migration arid the total displacement of the pattern can be calculated providing a measure of the angular velocity and relative angular position of the torque input members. A "home mark" on the surface of one of the grating elements, as described in reference to
Array 20 (or arrays 63 and 64) receive incident EMR reflected from the regions of high reflectivity on the surfaces of grating elements 58 and 59 which are instantaneously in the array's (or arrays) viewing window. In the example shown in
Most importantly however, in both of the bar code embodiments described in
Firstly, for still larger relative displacements of grating element 58 with respect to grating element 59, the problem of aliasing is avoided. This is readily demonstrated in the case of the bar code embodiment shown in
where r is the radius of the grating element.
Without the use of bar codes, aliasing would have occurred for a much smaller relative displacement of the two grating elements, namely when bar code 60b became adjacent to bar code 61c. By arranging the regions of high and low reflectivity in the former of successive binary bar codes, relative angular displacements of grating elements 58 and 59 can be correctly measured for substantial deviations from the zero torque condition, and independent of the actual spacing of the regions of high and low reflectivity. Secondly, use of bar codes enables measurement of the absolute angular position of either of the two torque input members 1a and 1b through a range of 360 deg, that is +/-180 deg from some known absolute position. This is achieved without the need for any counting process to be carried out in processor 9 (as in the case of the previous alternative version of the third embodiment of the present invention shown in FIG. 19).
If the maximum relative angular displacement of the grating elements is externally limited in some manner, for example via the use of a failsafe mechanism as earlier described, aliasing may not be a problem and in this case successive bar codes may be employed on only one of the two grating elements. This will still provide sufficient information to provide a measure of absolute angular position over the above mentioned +/-180 deg range.
It should be noted that the use of bar codes can be similarly applied to other cylindrical reflective grating element configurations, for example that described in reference to the first embodiment of the present invention (refer to FIGS. 1 and 2). Also barcodes can be applied to radially disposed reflective grating element configurations, for example those described in reference to the fourth (
Also, it should be noted that bar codes can take many forms apart form the non-overlapping (discrete) barcode arrangement shown in
It should also be noted that the succession of bar codes could have reverse reflectivity compared to the embodiment described, that is low reflectivity regions imposed over a high reflectivity background, rather than the other way around as described. Also in the present specification "high reflectivity" and "low reflectivity" is broadly defined in reference to the particular EMR source selected. For example, if a red light EMR source was used, the regions of high and low reflectivity of the surfaces of the reflective gratings may consist of regions which are painted (or otherwise coloured by some means) with a red and blue surface coating respectively.
Lastly it should be noted that the surfaces of the reflective grating elements may have forms other than the cylindrical or disc-like forms described by way of the above mentioned embodiments. Specifically the surfaces of the grating elements can have other three-dimensional axi-symmetric forms about the axis of the shaft, for example conical, elliptoidal, or paraboloidal forms. Any arbitrary axi-symmetric form of surface can potentially be used providing that the deviation of the distance between the surface and the respective array (receiving incident EMR from this surface) is sufficiently small in magnitude, that the aforementioned lens or fibre optic light guide system can maintain a satisfactory level of focus of the patterns (or subpatterns on the array).
It will be appreciated by those skilled in the art that numerous variations and modifications may be made to the invention without departing from the spirit and scope of the invention.
Eisenhauer, Karl Yarnos, Baxter, John
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